One
of the most important factors in the efficiency of any HVAC system is
the selection and implementation of the proper control sequence of
operations. Control sequences are the responsibility of the
systems designer and are used to describe how systems shall
operate. Key elements of any sequence include:

Reliable Operations: Detecting if a system element needs to be
repaired or replaced and starting a backup unit, if available.
Examples of this include detecting fan and pump status, and looking at
the pressure drop across a filter.

Comfort and Safety: This is the main reason for any
sequence! Examples of this include maintaining proper setpoints,
managing outdoor air, tracking and controlling indoor environmental
quality.

Efficiency: This may be the most difficult part of any
sequence. Efficiency looks at many issues beginning with
ventilation levels, use of economizers, preventing simultaneous heating
and cooling, resetting temperature and static pressure settings, and
coordination across systems such as chiller plants.

The process typically used to develop a control sequence starts with
the systems design engineer who needs to consider the owner's
requirements, codes and standards, systems selected, and then select
and specify the sequence as part of an overall controls system
design. Sequences are typically described in a written format
that becomes part of the project specifications. Controls
contractors will typically review these sequences when bidding a
project, and again when developing the project submittals. A
controls technician, working for the contractor, needs to interpret
these written sequences, and express them in the programming language
used as part of the controls system. Ideally, sequences are then
tested and reviewed during project implantation and
commissioning.

Along the way, there are many things that can, and generally do, go
wrong with control sequences. This can begin with challenges in
developing the sequences and can continue with how the contractor
interprets them and completes their implementation. While most
systems generally are able to achieve the goals for equipment
protection, reliability and comfort, achieving optimal efficiency is a
challenge due to difficulties that start in design and continue through
the contracting process.
ASHRAE Guideline 36P:

One potential solution for better design of control sequences is coming
from the proposed ASHRAE Guideline 36 titled “High Performance Sequence
of Operations for HVAC Systems”. This project has included
committee work to define a series of optimized sequences as well as
their associated points list and functional performance tests.
The work being done on this project is very important for the industry
and can provide the starting point for a greatly improved process for
the development, testing, implementation and validation of optimized
control sequences.

About the Authors

Paul and Ira first
worked together on a series of ASHRAE projects
including the BACnet committee and Guideline 13 – Specifying DDC
Controls. The formation of Building Intelligence Group provided them
the ability to work together professionally providing assistance to
owners with the planning, design and development of Intelligent
Building Systems. Building Intelligence Group provides services for
clients worldwide including leading Universities, Corporations, and
Developers. More information can be found at
www.buildingintelligencegroup.com
We also invite you to contact
us directly at Paul@buildingintelligencegroup.com or
ira@buildingintelligencegroup.com